1. Field of the Invention
This invention generally relates to dosimeters and more particularly to dosimeters used for monitoring ion implantation processes.
2. Description of Related Art
Ion implantation devices, such as shown in U.S. Pat. No. 5,760,409 (1998) to Chen et al., require accurate measurements of ion beam characteristics such as ion beam current and charge. Atoms of an ion beam carry known charges, so such measurements provide important information for controlling the atomic dose implanted into a work piece, such as a semiconductor substrate. Faraday cages, or cups, are standard devices that trap and measure the ion beam charge, current or both. An electronic dose processor measures the Faraday charge or current to assess the implanted ion dose.
Ion beam currents from a Faraday cup and currents from other sensors can have a wide dynamic range. For example, U.S. Pat. No. 4,963,747 (1990) to Thacker discloses one type of ionizing radiation detector that integrates the input current from a detector in the sub picoampere to nanoampere range. This particular detector, that provides a qualitative indication of radiation above a particular threshold, can handle currents corresponding to radiation from 1000 to 0.01 R/hr. This is a dynamic range of 100,000:1.
U.S. Pat. No. 4,721,857 (1988) to Kronenberg discloses a sampling and recording dose rate meter that operates over a range 30 from 10,000 R/hr. In this instrument a sample and record circuit receives a selected input and transfers the result to a selected one of different storage capacitors for a precise interval of time. The plurality of storage capacitors covers the entire dynamic range. The selected storage capacitor accumulates and holds a voltage proportional to its charge for being read by an electrometer.
Circuits for monitoring low-level currents in other applications have also been provided. My U.S. Pat. No. 4,143,318 (1979) discloses a system that converts a signal from an ion gauge tube to a signal with a frequency proportional to the ratio of two current magnitudes, one of which is constant. Specifically, ion gauge tube current charges an integrating capacitor that is periodically discharged after the voltage across the capacitor reaches a pre-selected threshold. The variable frequency output signal is based upon the rate of discharges (the charge rate) and, since current is defined as rate of charge, is proportional to the current. Over any given interval, the integrated value of this variable frequency output signal, i.e., the pulse count, represents accumulated charge.
Similarly, U.S. Pat. No. 4,083,044 (1978) to Marshall et al. discloses a unipolar wide range current-to-frequency converter. This particular converter sums an input current and a feedback current. The input current represents the ion beam current from a Faraday cage or like device and a capacitive feedback amplifier produces a charge signal. When the charge signal reaches a predetermined level, a gated multivibrator enables a charge pulser to reduce the signal at the input by fully discharging the capacitive feedback amplifier. A data processor converts the number of charge pulser activations to a measurement of the accumulated charge.
Requirements for such measurement systems, especially those used in such ion implantation systems, are becoming more stringent. They require more frequent measurements than can accurately be made by accumulating slow pulses at low currents that are characteristic of prior art dosimeter systems. For example, it now is desirable to provide a measuring resolution of xc2x11% or xc2x13 pC, whichever is greater, of charge over a dynamic range of ion beam currents from below 100 nA to over 50 mA at measurement intervals from a few microseconds to several seconds.
Current-to-frequency converter measurements such as disclosed in the Marshall et al. and my patents have certain characteristics that can be disadvantageous in such applications. Each discharge removes an incremental charge. The resolution of the output signal is therefore dependent upon the magnitude of that incremental charge. Moreover, as the magnitude of the incremental charge decreases for better resolution, the discharge frequency at a given current will increase. With the wide dynamic ranges encountered in ion implantation systems, this frequency conversion approach has practical limits because circuits with such wide frequency ranges are difficult to manufacture and calibrate for use. If a current measurement rather than a charge measurement is desired, then, at very low currents, one has to wait for at least two pulses to make a frequency measurement. Further, in the Marshall et al. patent, each activation of the charge pulser fully discharges the charge accumulator capacitor. The final measurements then have an accumulated error corresponding to the sum of the charge represented by charging current that is simultaneous with each discharge.
Therefore it is-an object of this invention to provide a dosimeter for an ion beam that accurately measures accumulated charge over time.
Another object of this invention is to provide a dosimeter for accurately measuring accumulated charge over time and over a single wide dynamic range without range switching.
Yet another object of this invention is to provide a dosimeter for ion beam current for accurately measuring accumulated charge over time and over a single wide dynamic range that is practical to manufacture.
Still another object of this invention is to provide a dosimeter for accurately measuring accumulated charge over time and over a single wide dynamic range that is simple to calibrate.
Still another object of this invention is to provide a dosimeter for accurately measuring accumulated charge over a wide dynamic range using a high sampling rate.
A dosimeter for measuring accumulated charge in accordance with this invention includes a charge accumulator having an input for receiving a signal and an output for producing a signal representative of accumulated charge. A charge measurement unit measures the magnitude of the accumulated charge at the charge accumulator output. A charge removal unit at the charge accumulator input removes a fixed charge from the charge accumulator when the magnitude of the accumulated charge reaches a predetermined threshold. Combining the number of operations of the incremental charge removing unit and the indicated charge magnitude from the charge measurement unit yields the accumulated charge.
In accordance with another aspect of this invention a dosimeter for measuring charge comprises a charge accumulator having an input and an output for producing a signal representative of accumulated charge at the input. The charge accumulator includes an operational amplifier with an input and output, a feed back capacitor for the operational amplifier that stores a charge and a summing junction at the operational amplifier input. When the magnitude of the accumulated charge at the charge accumulator output reaches a threshold, a charge removal circuit removes a fixed charge quantity from the charge accumulator. The charge removal circuit includes a switched capacitive storage circuit for storing a fixed charge and has charging and discharging modes of operation and a circuit for shifting the switched capacitive storage circuit between the charging and discharging modes of operation. The switched capacitive storage circuit switches to its discharge mode to discharge a fixed charge from the charge accumulator input when the signal at the charge accumulator output reaches the predetermined threshold value. The number of operations of the charge removal circuit and the indicated charge magnitude together correspond to the accumulated charge.
In accordance with still another aspect of this invention a dosimeter for measuring charge accumulated at a Faraday cup in an ion implantation device comprises a charge accumulator operational amplifier with an input, an output and a feed back capacitor for storing charge and a summing junction at the operational amplifier input for connection to the Faraday cup. A charge measuring analog-to-digital converter connects to the charge accumulator operational amplifier output for generating a digital output. A charge removal switched capacitive storage circuit for storing a fixed charge connects to the charge accumulator operational amplifier input and has charging and discharging modes of operation. A charge removal threshold circuit shifts the charge removal switched capacitive storage circuit between the charging and discharging modes of operation whereby the switched capacitive storage circuit discharges the fixed charge from the input of the charge accumulator operational amplifier when the signal at the charge accumulator operational amplifier output exceeds a predetermined threshold value. Combining the number of operations of the charge removal switched capacitive storage circuit with the output of the charge measuring analog-to-digital converter yields the accumulated charge.
In accordance with still another aspect of this invention a dosimeter for measuring the net charge comprises a charge accumulator, a charge measurer, first and second incremental charge modifiers and a counter circuit. The charge accumulator has an input connected to the charge gatherer and an output for producing a signal representative of accumulated charge. The charge measurer measures the magnitude of the accumulated charge at the charge accumulator output. A first charge modifier at the charge accumulator input removes a fixed charge from the charge accumulator during a single operation when the charge in said charge accumulator exceeds a first predetermined threshold. The second charge modifier at the charge accumulator input adds a fixed charge to the charge accumulator during a single operation when the charge in the charge accumulator falls below a second predetermined threshold. Two counter circuits provide a net count of the number of operations of said first and second charge modifier whereby the two counts and the indicated charge magnitude yield the net charge accumulated in said charge gatherer.